Yersinia pestis, the causative agent of plague, secretes LcrV (low calcium response V or V antigen) during infection. LcrV triggers release of interleukin 10 (IL-10) by host immune cells and suppresses pro- inflammatory cytokines such as tumor necrosis factor alpha (TNF-a) and interferon-gamma (IFN-Q) as well as innate defense mechanisms required to combat the pathogenesis of plague. Further, LcrV plays an important role in the type III secretion machinery-dependent delivery of Yop virulence factors into host cells as a bacterial means to combat innate immune responses. Although immunization of animals with LcrV elicits protective immunity, the associated suppression of host defense mechanisms may preclude the use of full length LcrV as a human plague vaccine. Herein we show that short deletions within LcrV can reduce its immune modulatory properties. An LcrV variant lacking amino acid residues 271-300 (rV10) elicited immune responses that protected mice against a lethal challenge with fully virulent Y. pestis strain CO92. When compared to full-length LcrV, rV10 immunization afforded greater levels of vaccine protection in a murine model of pneumonic plague. In contrast to full length LcrV, rV10 displayed reduced ability to release IL-10 from mouse and human macrophages. Further, LPS-stimulated release of pro-inflammatory cytokines by human or mouse macrophages was inhibited by full length LcrV but not by the rV10 variant. Thus, because the rV10 variant displays significantly reduced immune-modulatory properties and superior immunogenicity, rV10 can likely be used as a safe human vaccine to generate protective immunity against plague. This hypothesis will be pursued by an inter-disciplinary team of scientists at the University of Chicago, the Great Lakes RCE for Biodefense & Emerging Infectious Diseases, Cambrex Inc. and SRI International. Product development research is structured to achieve four successive milestones. Following development of GLP large scale production of rV10 (Milestone 1) and GMP grade vaccine preparation (Milestone 2), rV10 vaccine immune responses and safety will be determined (Milestone 3). Using plague challenge of immunized animals, FDA-CBER Animal Rule regulations will be followed to examine vaccine efficacy (Milestone 4). Research protocols will be designed in close collaboration with FDA-CBER and the entire body of acquired information will be used to develop rV10 vaccine as an Investigative New Drug, permitting its future use in human clinical trials. [unreadable] [unreadable] [unreadable]